1. Field of the Invention
This invention relates generally to corona discharge gas sensors and, more particularly, to an improved gas sensor which incorporates an integrating filter for purposes of stabilizing the sensors and eliminating the deleterious effects of water vapor or high humidity regions which impede the detection of leaks.
2. Description of the Prior Art
Refrigerants such as freon are generally utilized in air conditioning and refrigeration systems. In the installation and maintenance of these systems, a method and instrument for detecting gas leakage from the systems was a practical requirement to initially test the proper installation, as well as to maintain the system's operation. Large leaks completely immobilize the units; small leaks gradually decrease the efficiency over time, creating higher operating costs, and ultimately leads to total failure.
There are numerous known methods and devices for conveniently detecting the leakage of small quantities of refrigerant gases into the surrounding atmosphere and the negative corona electrical sensing devices have been found to be superior to other devices. Typically, such halogen or freon gas leak detectors utilize a sensing tip which is projected into the ambient atmosphere zone to be tested or sensed for freon gas contamination indicative of leakage. The sensing tip comprises a small protective shell, constituting the anode electrode, surrounding a point cathode energized by the hand-held body of the instrument to establish a negative corona within the protective shell. Slots or holes provided in the protective shell allow diffusion of the gas molecules into the corona discharge for effecting corona current changes in the presence of freon gas molecules. These current changes activate audio or visual alarms, indicating the presence of atmospheric halogen or gas contamination or leakage to the operator.
U.S. Pat. No. 4,609,875, entitled "Corona Discharge Freon Gas Sensor Having Electrical Wind Pumping Action", and U.S. Pat. No. 4,488,118, entitled "Halogen Gas Leak Detector", disclose sensors utilizing the corona discharge phenomenon to detect leakage. In these devices, electrical circuitry is utilized to produce a high voltage corona, which then reacts to changes in the presence of various gases to produce current changes. Such current changes are then monitored to produce alarm conditions.
Although very sensitive and generally efficient, such corona discharge sensors are also sensitive to water vapor. Condensation in and around air conditioning and refrigerant systems produces areas of high water vapor content relative to areas where there is no condensation or ambient atmosphere. When maintenance personnel or servicemen are searching for leaks, they can quickly move, for example, from an area of 50% relative humidity to one of 90% or higher. When this occurs, modern corona discharge driven detectors are sensitive enough such that they respond to the rapid change in humidity alone and produce a false alarm indication of a leak. That is, the detectors cannot distinguish between corona discharge current being affected by a small refrigerant leak or, alternatively, a rapid change in humidity.
As the CFC and HCFC refrigerant gases generally produce, even in small concentrations, a larger change in the corona discharge current than that affected by the presence of water vapor, servicemen have learned to ignore such false signals. This, of course, leads to human errors in judgment, is an extremely uncertain process, but one that has been generally overlooked by the industry. Also depending upon the size of a refrigerant leak, and its location, it can be impossible to locate the source of the leak when in areas of high water vapor.
This problem has now been compounded by the ozone layer depletion and the use of alternative refrigerants. CFC refrigerants are being replaced with less damaging HFC refrigerants, which are much more benign refrigerants. HFC refrigerants (R-134A) are also much more difficult to sense with a corona discharge detector. Such refrigerants have approximately 4 times less effect on the corona discharge current than do CFC refrigerants of the same concentration. The sensitivity of the detectors can be increased by this factor of 4, however, the detector is then 4 times more sensitive to changes produced by water vapor. This further makes it impossible to distinguish between small concentrations of HFC refrigerant gases and changes in humidity as small as 15%. Preliminary tests have shown that a 15% change in relative humidity will produce the same response from a corona discharge detector as a concentration of 100 parts per million (ppm) of R-134A refrigerant. A concentration of 100 ppm R-134A produces approximately the same response in corona discharge current as does a leak rate standard set at 1/2 ounce per year.
It is, therefore, highly desirable to provide an improved corona discharge gas detector which overcomes the deficiencies of conventional devices and eliminates false alarm conditions caused by changes in levels of water vapor.